Ideen TaebJon Mah

Combination of photonics and microelectronics

Advantages: Capacity to generate, transport and manipulate data at very high rate

Photonics/Optoelectronics refer to coexistence of electron and photons in the same system

First transmission trunk using glass fibers in 1983

Photon law is tripling the bandwidth every year.

Compared to copper wire, optical fibers cost less, weigh less, have less attenuation and dispersion and provide more bandwith.

Highly used in electronicsystems

Growing every year, 30% growth every year since 1992

Combined market for optoelectronic components and final end-products currently stands at $30 billion

Electron vs. Photon◦ Mass◦ Charge◦ Spin◦ Pauli Exclusion Principle◦ Velocity

LED- Advantage of ease of use, 160 degrees circular cone light emission, but low in power

LD- Advantage of high power around 30 mW, but emission in elliptical cone rather than circular.

VCSEL- Have both high power as wells as emission into circular cone, furthermore they can be produced in uniform arrays on wafers

Forward biased junction Current flows from n side to the p side Band Gap or Energy Gap (EG): Difference of

energy between the conduction band and valance band

Wavelength of emitted light depends on EG Most widely used material for visible

spectrum: GaAs, GaP, and GaAsP

Forward biased p-n junctions where emmited photons are confined in an optical cavity

Two types◦ Edge Emitting: Wide, astigmatic emission◦ Surface Emitting: Narrower Beam Emission

Different from LDs and LEDs, light emission occurs in a direction perpendicular to the active region

Have a potential to be operated at orders of Gb/s speed

P-i-n Photodiodes◦ A p-n junction with a

sandwiched intrinsic layer

◦ Operated in the reverse-biased mode

◦ Response times are in order of 10 ps.

MSM Detectors◦ Consists of two

interdigitated electrodes which form back to back schottky diodes.

◦ Very fast, can be switched completely on or off with an applied bias

◦ Response time in in order of 1ps

Free-Space Channels◦ High-speed communication (>1Gbs)◦ Wide BW, elimination of impedance mismatch problem◦ Potential for high density interconnects◦ Decreased interconnection delays and so on

Disadvantages caused by:◦ Potentially require a significant change in the way system

architectures are designed◦ Laser wavelength stabilities in the order of 1nm can be

expected(Dispersion)◦ Physical size of some proposed architectures are

prohibiting◦ Power inefficiencies can be limiting◦ Dependent on weather

Guided Wave Channels◦ Can be classified according to the interconnection

medium employed and the level of interconnection hierarchy they target

Speed of propagation in a medium

ncv

hcE

• Photon Energy

• Frequency

cf 22

Speed of EM waves in a medium depends on interactions with Electric Field and Magnetic Field

material

vacuumc

cn

)sin()sin( 2211 nn

Critical Angle (Φ1) occurs at Φ2=90˚

1

2sin nnacritical

• For angles larger than the critical angle, have total internal reflection (TIR)– Principle behind traditional waveguides

• different from photonic crystal waveguides

– Phase changes with the angle

n1 > n2, but just barely

Then NA is small 22

21sin nnnNA acc

Light of different frequencies propagate at different speeds through the medium◦ Typical units of ps/nm-km

Due to both material (n = n(λ)) and waveguide effects (effective n1, n2)

Birefringence caused by Polarization Effects (fiber cross section not perfectly circular).

Higher order effects (Kerr effect)2KEnnn perppar

Due to imperfections in fabrication as well as Rayleigh Scattering◦ Scattering due to particles smaller than λ

(why is the sky blue?)◦ Units of dB/km◦ For GeO2-doped single-mode silica fiber

~0.2dB/km at λ=1.55μm

)0(

)(log

1010 P

LP

L

• Also get attenuation due to bending

Time-Division Multiplexing (TDM)◦ E.g. Telephone lines

Frequency-Division Multiplexing◦ E.g. FM radio

Wavelength-Division Multiplexing◦ Optical effect◦ E.g. Prism

Superprisms made from Photonic Crystals (large dispersion in periodic media)

Fused Silica (SiO2) Fiber Can be made extremely pure, then doped to attain

desired n Exhibits very low loss and dispersion at λ=1.55μm

Plastic Fiber Lossy (~102 dB/km) Flexible, inexpensive, lightweight

Other Glass Fiber Chalcogenide, fluoroaluminate, etc. for longer

wavelengths

Major problems in coupling fiber1. The fibers must be of compatible types

Dispersion effects, single mode/multi mode

2. The ends of the fiber must be brought together in close proximity

Matching of NA

3. The fibers must be accurately aligned with eachother

corelaunching

corereceiving

Diam

DiamLoss

_

_10log20

Bragg Gratings:constructive interference

where d=distance between gratings

sin2dn

Optoelectronics market is growing every year Optoelectronics provide a high bandwidth for

communications Utilize TIR for light propagation in waveguides Dispersion and attenuation are main drivers

in optical fiber design Interconnections and coupling require precise

alignment of optical elements A number of inter- and intra-chip connection

schemes exist and are being explored